The field of implantable defibrillation devices is well known. Implantable defibrillators monitor cardiac signals from a patient's heart and, in response, deliver an electrical therapy or countershock in the event that a life threatening ventricular fibrillation is detected. The primary goal of any ventricular defibrillator device is to deliver a countershock that effectively defibrillates the ventricles of the heart, thereby resuscitating the patient from a life threatening condition. While it is known that defibrillation can be accomplished with energies as low as a few joules for an implantable defibrillator, these devices must be designed with the capacity to deliver electrical countershocks of significantly larger energies. There are several reasons for this design constraint, but the primary reason is that, because ventricular fibrillation is a largely chaotic condition, it can take widely different amounts of energy to defibrillate different episodes of fibrillation even in the same patient. Consequently, an implantable defibrillator must be designed to provide a maximum energy defibrillation countershock that has the greatest probability of defibrillating a patient within given parameters.
It is well known to synchronize the delivery of a cardioversion or defibrillation countershock to some aspect of the R-wave that represents the ventricular activity of the heart. If no R-wave is detected within a given time period, then the defibrillation countershock is delivered asynchronously into the heart. Examples of R-wave synchronization schemes are shown in U.S. Pat. Nos. 3,950,752, 4,384,585 and 5,193,536. One of the rationales behind synchronizing the defibrillation countershock to an R-wave is based on the idea that the R-wave as sensed by the defibrillator would represent the largest contingent of organized cells within the heart at that given moment that would not be in a refractory condition and thereby more difficult to stimulate. If this theory is correct, then synchronizing to the R-wave should be the point in time requiring the lowest energy in order to defibrillate the heart. While the theory behind this practice seems sound, the clinical experience of such wide variabilities in defibrillation thresholds when using this technique does not appear to substantiate the theory.
Other approaches to timing of delivery of a defibrillation countershock to some aspect of the R-wave signal have been proposed. In U.S. Pat. No. 5,578,062, the idea of synchronizing delivery of a defibrillation countershock to a time delay equivalent to 50% of a measured R-R interval is disclosed. In U.S. Pat. No. 5,531,767, the timing of the defibrillation pulse is synchronized to the peaks and troughs of the ECG signal as detected from a far-field detector. In U.S. Pat. No. 5,545,182, the timing of the defibrillation pulse is synchronized to a point in time at which the fibrillation the ECG signal has substantially its highest amplitude and lowest frequency. Other authors have suggested synchronizing the delivery of a defibrillation countershock to various features of the sensed ventricular activity of the heart, such as the minimum or maximum derivative of the ventricular signal. Li et al., "Effect of Shock Timing on Efficiency and safety of Internal Cardioversion for Ventricular Tachycardia," JACC, Vol. 24, No. 3, Sept. 1994, pp. 703-708 (cardioversion shocks synchronized to QRS complex+100 ms); Hsu, et al., "Effect of Shock Timing on Defibrillation Success", PACE, Vol. 20, January 1997, pp. 153-157 (delivery of defibrillation shocks on upslope of ventricular signal more effective); Kuelz et al., "Integration of Absolute Ventricular Fibrillation Voltage Correlates with Successful Defibrillation", IEEE Trans. Biomed. Engr., Vol. 41, 1994, pp. 782-791 (delivery on defibrillation shocks more likely to be successful when the absolute ventricular fibrillation voltage was high than when the absolute ventricular fibrillation voltage was low). Unfortunately, all of these approaches rely on the sensed ventricular signal to provide the necessary information from which a decision will be made as to the timing of the delivery of the defibrillation countershock.
In U.S. Pat. No. 5,431,687, the idea of synchronizing delivery of a defibrillation countershock to a low value of a detected impedance of the electrode leads is disclosed. The theory behind this idea is that delivered current is responsible for effective defibrillation and the lower the measured impedance between the electrodes, the higher the delivered current will be. While this idea again seems sound in theory, to date no correlation has been established between minimum inter-electrode resistances and minimum defibrillation thresholds.
In U.S. Pat. No. 5,279,291, an atrial defibrillator is described in which delivery of an atrial defibrillation pulse is synchronized by sensing depolarization waves at a first and second area of the heart and synchronizing the atrial defibrillation pulse to the sensed depolarization waves. For an atrial defibrillator of the type described in this patent, synchronization of delivery of the atrial defibrillation pulse is critical so as not to shock into T-wave of the cardiac signal, something that is known to induce ventricular fibrillation. U.S. Pat. No. 5,403,354 improves on this technique by using a three channel sensing arrangement rather than the two channel sensing arrangement as described in the previous patent.
In U.S. Pat. No. 5,713,924, an atria defibrillator is described that treats atrial fibrillation using a high frequency, low energy pulse delivered via pacing electrodes, followed by a high energy pulse delivered via defibrillation electrodes. The delivery of this atrial therapy is synchronized to the R-wave. The patent also discusses a co-pending application in which treatment of atrial fibrillation with just a high frequency pulse burst to the atrium is synchronized to atrial depolarizations or the P-wave.
In U.S. Pat. No. 5,074,301, delivery of a ventricular cardioversion or defibrillation countershock in a dual chamber defibrillator is synchronized to a delay period following delivery of a pacing pulse to the atrium. The synchronization of the ventricular countershock based on a predetermined delay from an atrial pacing pulse is claimed to minimize post-shock atrial arrhythmias due to the possibility that the ventricular countershock was delivered during a vulnerable period in the atrium. The forced delivery of an atrial pacing pulse produces an atrial refractory period during which the atrium is not susceptible to atrial arrhythmias.
While the goal of reducing defibrillation thresholds in terms of the maximum energy required for consistently successful ventricular defibrillation is a long recognized goal, the existing techniques for synchronizing the delivery of ventricular defibrillation countershocks could be improved upon to decrease the variability of energy required for successful defibrillation.